The present invention relates to a low-cost high-durability and high oxidation-resistant solid polymer electrolyte fit for an electrolyte membrane used for fuel cells, electrolysis of water, electrolysis of halogenated hydracid, electrolysis of salt (solution), oxygen concentrators, humidity sensors, and gas sensors, a solid polymer electrolyte membrane using thereof, a solution for covering electrode catalyst, a membrane/electrode assembly, and a fuel cell.
A solid polymer electrolyte is a solid polymer material having electrolytic groups such as sulfonic groups in polymer chains and has features of strongly bonding to specific ions and selectively permeating cations or anions. The solid polymer electrolyte is formed into particles, fibers or thin films and used for electrodialysis, diffusion dialysis, cell diaphragms, and so on.
A reformed gas fuel cell comprising a cathode, an anode, and a proton conducting solid polymer electrolyte membrane sandwiched between these electrodes supplies a hydrogen gas obtained by reforming hydrocarbons of low molecular weights such as methane and methanol as a fuel gas to one electrode (fuel electrode) and an oxygen gas or air as an oxidizing agent to the other electrode (air electrode), and obtains electromotive forces by their reactions. Electrolysis of water electrically decomposes water by a solid polymer electrolyte membrane into hydrogen and oxygen.
Fluorine-related electrolyte membrane such as perfluorocarbon sulfonic membrane having high proton conducting show very high long-term chemical stability as a solid polymer electrolyte membrane for fuel cells and water electrolysis. Typical products of the fluorine-related electrolyte membrane are Nafion (trademark of DuPont), Aciplex (trademark of Asahi Chemicals Co., Ltd.) and Flemion (trademark of Asahi Glass Co., Ltd.)
Electrolysis of a salt solution electrically decomposes a water solution of sodium chloride by a solid polymer electrolyte membrane into sodium hydroxide, chlorine, and hydrogen. As the solid polymer electrolyte membrane. In this case, the electrolyte membrane is in contact with a chlorine gas and a hot and concentrated water solution of sodium chloride and must be resistant to them. Therefore hydrocarbon-related electrolyte membranes are not available. In general, perfluorocarbon sulfonic membranes having carboxylic groups partially on its surface to prevent inverse diffusion of ions are used as solid polymer electrolyte membranes which is resistant to chlorine gas and hot and concentrated alkaline water.
Basically, the fluorine-related electrolyte represented by carbon sulfonic membranes is very high chemical stability due to Cxe2x80x94F bonds. The fluorine-related electrolyte membranes are used not only as solid polymer electrolyte membranes for fuel cells, water electrolysis, or salt electrolysis but also as solid polymer electrolyte membranes for electrolysis of halogenated hydracid. Due to its high proton conducting, the fluorine-related electrolyte membranes are also used for humidity sensors, gas sensors, oxygen concentrators, and so on.
Contrarily, the fluorine-related electrolyte membranes are hard to be manufactured and very expensive. So their use is much limited for example, to solid polymer electrolyte fuel cells for space and military fields and to other particular uses. They are hard to be used for solid polymer electrolyte fuel cells as low-pollution power sources for automobiles and other public uses.
So various aromatic hydrocarbon electrolyte membranes as inexpensive solid polymer electrolyte membranes have been disclosed such as sulfonated poly-ether ether ketone by Japanese Non-examined Patent Publications No.H06-93114 (1994), sulfonated poly-ether sulfone Japanese Non-examined Patent Publications No.H09-245818 (1997) and Japanese Non-examined Patent Publications No.H11-116679 (1999), sulfonated acrylonitrile butadiene styrene monomer by Japanese Non-examined Patent Publications No.H10-503788 (1998), sulfonated poly sulfide by Japanese Non-examined Patent Publications No.H11-510198 (1999), and sulfonated polyphenylene by Japanese Non-examined Patent Publications No.H11-515040 (1999). The aromatic hydrocarbon electrolyte membranes prepared by sulfonating engineer plastics are easier to be manufactured and lower costed than fluorine-related electrolyte membranes represented by Nafion. However, one of the demerits of the aromatic hydrocarbon electrolyte membranes is to be easily deteriorated. This reason is revealed by Japanese Non-examined Patent Publications No.2000-106203. It says the main reason is that the structure of aromatic hydrocarbon is oxidized and broken by hydrogen peroxide which generates in the catalyst layer on the boundary between the solid polymer electrolyte membrane, and the air electrode (oxidant electrode).
So various trials have been made to prepare a solid polymer electrolyte which is as resistant to oxidation as the fluorine-related electrolyte membrane or stronger and which can be manufactured at low costs. For example, Japanese Non-examined Patent Publications No.H09-102322 (1997) proposes a membrane made of sulfonic type polystyrene graft ethylenetetrafluoroethylene (ETFE) co-polymer having a hydrocarbon-related side chain and a main chain formed by copolymerization of fluorinecarbide-related vinyl monomer and hydrocarbon related vinyl monomer. This polystyrene-graft-ETFE membrane (aromatic hydrocarbon polymer) is not expensive and strong enough as a solid polymer electrolyte membrane for fuel cells. Its conductivity can be improved by increasing the quantity of sulfonic groups to be attached. However, the side chains of this membrane having sulfonic groups are easily subject to deterioration by oxidation although the main chains formed by copolymerization of fluorinecarbide-related vinyl monomer and hydrocarbon related vinyl monomer. Therefore, this polystyrene-graft-ETFE membrane as a total is not resistant to oxidation and not so durable. Consequently, this polystyrene-graft-ETFE membrane is not available to fuel cells.
U.S. Pat. Nos. 4,012,303 and 4,605,685 propose a sulfonic poly-(trifluorostyrene) graft ETFE polymer electrolyte membrane which is prepared by copolymerizing fluorinecarbide-related vinyl monomer and hydrocarbon related vinyl monomer, grafting xcex1, xcex2, xcex2-trifluorostyrene with the resulting membrane, and attaching sulfonic groups thereto. This membrane uses xcex1, xcex2, xcex2-trifluorostyrene which is partially fluorined instead of styrene because the polystyrene side chain having sulfonic groups is not chemically stable. However, it is very difficult to synthesize xcex1, xcex2, xcex2-trifluorostyrene which is material of the side chains. Further, the material as well as Nafion is too expensive to be used as solid polymer electrolyte membranes for fuel cells. Furthermore, xcex1, xcex2, xcex2-trifluorostyrene has low reactivity of polymerization and consequently the quantity of xcex1, xcex2, xcex2-trifluorostyrene to be grafted for side chains is very small. The conductivity of the resulting membrane is very low.
It is an object of the present invention to provide an easy manufacturability high-durability solid polymer electrolyte which is as durable as the fluorine-related electrolyte or has substantially high chemical stability, a solid polymer electrolyte membrane using thereof, a solution for covering electrode catalyst, a membrane/electrode assembly, and a fuel cell.
To dissolve the aforesaid problems, we inventors researched the mechanism of deterioration of electrolyte membranes and found that the main cause of the deterioration of the aromatic hydrocarbon electrolyte membranes is not the deterioration by oxidation but rather the direct bonding of a sulfonic group to an aromatic ring. This direct bonding allows the sulfonic group to be easily cut out from the aromatic ring in the presence of a strong acid at a high temperature and as the result, causes reduction of its ionic conductivity. Judging from this result, the high-durability solid polymer electrolyte in accordance with the present invention is an aromatic hydrocarbon polymer having a sulfoalkyl group (FORMULA 1) instead of a sulfonic group in the side chain. The present invention can provide low-cost high durability solid polymer electrolyte which is as durable as the fluorine-related electrolyte or has substantially high chemical stability, Further, the ionic conductivity of the electrolyte having the sulfoalkyl groups in the side chains is greater than the ionic conductivity of the electrolyte having the sulfonic groups in the side chains (per weight equivalent to ion exchange group), it is assumed that is related to that the sulfoalkyl groups can move more freely than the sulfonic groups.
Said aromatic hydrocarbon polymer compound is preferably poly-ether sulfone polymer compounds, poly ether ether kletone polymer compounds, polyphenylene sulfide polymer compounds, polyphenylene ether polymer compounds, poly-sulfone polymer compounds, or poly ether ketone polymer compounds.
It is preferable that the polymer electrolyte membrane and the solution for covering electrode catalysts contain said polymer electrolyte.
In accordance with the present invention, it is preferable that a membrane/electrode assembly for a solid polymer electrolyte fuel cell comprises a polymer electrolyte membrane and a gas diffusion electrode unit comprising a cathode and an anode which are placed on both sides of said polymer electrolyte membrane, wherein said polymer electrolyte membrane is any polymer electrolyte membrane stated above, said gas diffusion electrodes bind fine catalytic metal particles to the surfaces of a conductive material made of carbon with a binder, and said binder is made of any polymer electrolyte stated above.
In accordance with the present invention, it is preferable that a solid polymer electrolyte fuel cell comprise a polymer electrolyte membrane, one pair of gas diffusion electrodes comprising a cathode and an anode which are placed on both sides of said polymer electrolyte membrane, one pair of gas impermeable separators which are provided to sandwich said gas diffusion electrodes, and one pair of current collecting members which are placed between said solid polymer and said separator, wherein said solid polymer electrolyte membrane is made of any polymer electrolyte membrane stated above and said polymer electrolyte membrane and said gas diffusion electrodes are made of said membrane/electrode assembly for a solid polymer electrolyte fuel cell.